Integrated photonic device with improved optical coupling

ABSTRACT

A three-dimensional photonic integrated structure includes a first semiconductor substrate and a second semiconductor substrate. The first substrate incorporates a first waveguide and the second semiconductor substrate incorporates a second waveguide. An intermediate region located between the two substrates is formed by a one dielectric layer. The second substrate further includes an optical coupler configured for receiving a light signal. The first substrate and dielectric layer form a reflective element located below and opposite the grating coupler in order to reflect at least one part of the light signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application patent Ser. No.15/377,848 filed Dec. 13, 2016, which claims the priority benefit ofFrench Application for Patent No. 1654523, filed on May 20, 2016, thedisclosure of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments relate to photonic integrated devices, and notably to thecoupling of this type of device with an external optical signal, comingfrom, for example, but not limited to, an optical fiber.

BACKGROUND

Conventionally, for coupling a photonic integrated circuit to an opticalsignal originating, for example, from an optical fiber, an opticalcoupler is implemented in the active layer of the integrated circuit forredirecting the light signal into a waveguide implemented in thestructure.

During coupling, one part of the input optical signal passes through thecoupler and is not transmitted in the waveguide. Means exist forimproving the efficiency of the coupling, such as optimizing thethickness of the buried insulating layer in the case of aSilicon-On-Insulator (SOI according to the abbreviation well known tothe person skilled in the art) substrate. However, even with an optimumthickness, a part of the signal is lost.

Another solution consists in placing a reflective layer, for example, ametal layer, under the buried layer, in order that the rays passingthrough the coupler are reflected and pass back again into the coupler.However, the production of such a metal layer requires specific methodsteps.

SUMMARY

Thus, according to one embodiment, provision is made here to furtherreduce the losses of an optical signal arriving at an integrated opticalcoupler.

In this respect, provision is advantageously made to use not a singleintegrated circuit but an integrated three-dimensional structurecomprising multiple stacked substrates (forming a monolithic structure),and to implement a reflector in one of its substrates, under the opticalcoupler.

This has the advantage of limiting the optical signal losses andproducing the reflector with existing manufacturing methods, such asetching and deposition of dielectric material.

According to one aspect, a three-dimensional photonic integratedstructure is provided including a first semiconductor substrateincorporating at least one first waveguide, a second semiconductorsubstrate incorporating at least one second waveguide, and at least oneintermediate region located between the two substrates and comprising atleast one dielectric layer; the second substrate comprises at least oneoptical coupler configured for receiving a light signal, and the firstsubstrate and said at least one dielectric layer comprising a reflectiveelement located opposite said at least one optical coupler capable ofreflecting at least one part of said light signal.

The reflective element comprises, for example, a portion of the firstsemiconductor substrate and a portion of said layer of dielectricmaterial.

Thus, implementing the reflective element in a substrate comprisingother photonic components avoids the need for a specific method step forobtaining the optical reflector.

According to one embodiment, the intermediate region may furthercomprise at least one additional semiconductor layer coated in thedielectric layer and located opposite the optical coupler, thereflective element further comprising said additional layer.

Preferably, the product of the thickness of the portion of the firstsemiconductor substrate and its refractive index and the product of thethickness of the portion of said layer of dielectric material and itsrefractive index are both approximately equal to a quarter of thewavelength of the light signal.

The first substrate and the second substrate may be semiconductor filmslocated on insulating layers, thus forming silicon-on-insulatorsubstrates. In this case, the intermediate region advantageouslyincludes the buried insulating layer on which the second substrate islocated.

According to one embodiment, at least one part of the reflectiveelement, for example, said portion of the first substrate, has athickness less than or equal to the thickness of the first substrate.Thus, said portion may be etched or left as it is.

In particular, the thickness of said portion of the first semiconductorsubstrate may correspond to the thickness of other photonic componentsimplemented in the first semiconductor substrate.

The optical coupler may be of a single polarization type, and in thiscase be coupled to a single waveguide, or of a polarization splittingtype and then be coupled to multiple waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will appear onexamination of embodiments of the invention, in no way restrictive, andthe accompanying drawings in which:

FIGS. 1, 3 and 4 illustrate a monolithic photonic structure;

FIG. 2 shows a grating coupler as a single polarization coupler;

FIG. 5 shows a grating coupler as a polarization splitting coupler.

DETAILED DESCRIPTION

FIG. 1 illustrates a monolithic photonic structure SPM. The photonicstructure SPM comprises a carrier substrate 1, on which twosilicon-on-insulator substrates 2 and 3 are implemented separated by anintermediate region INT and in which multiple photonic components areimplemented.

The structure also comprises an interconnection region (BEOL, “Back EndOf Line” according to the acronym well known to the person skilled inthe art) not represented here for the purposes of simplification.

An optical fiber, for example, may be attached on the upper face of thestructure SPM, delivering an incident optical signal L1 whereof thewavelength is, for example, close to one thousand three hundred and tennanometers.

Here, the input optical signal arrives at the structure at a low angleθ, for example, between eight and thirteen degrees.

The first SOI substrate 2 comprises a first substrate proper, orsemiconductor film 22, and a first buried insulating layer 21 (known bythe person skilled in the art under the acronym “BOX”, for BuriedOxide), here a layer of silicon dioxide conventionally having athickness of seven hundred nanometers.

The first buried insulating layer 21 is located here under the firstsemiconductor film 22, having, for example, here a thickness of threehundred nanometers.

Multiple photonic components are implemented by etching in the firstsilicon film 22, then coated in a first layer of dielectric material 23,here of silicon dioxide, so that the assembly formed by the firstsilicon film 22 and the first dielectric layer 23 has a thickness offour hundred and fifty nanometers.

The first substrate 22 notably comprises a first waveguide 24 and a setof active components, including, for example, a photodetector 25.

The second SOI substrate 3, implemented directly above the layer ofdielectric material 23 by molecular bonding, comprises a second buriedinsulating layer 31 of a thickness, for example, equal to one hundrednanometers, forming with the layer of dielectric material 23 theintermediate region INT, whereon the second substrate proper is located,or second semiconductor film 32, made of silicon, for example.

The second substrate 3 comprises photonic components etched in thesecond semiconductor film 32 and coated in a second layer of dielectricmaterial 33. Here, the components notably comprise a second waveguide 34optically coupled to an optical coupler 36 of a grating type.

The structure SPM also comprises a reflective element 26, here a Braggmirror conventionally formed by multiple layers having differentrefractive indices.

In this example, the Bragg mirror 26 includes two stacked layers,including a first layer formed by one portion 220 of the first siliconfilm 22, and a second layer formed by the stacking of one portion 230 ofthe first dielectric layer 23 and one portion 310 of the second buriedinsulating layer 31 of the second substrate 3.

The thicknesses of the two layers of the Bragg mirror 26 are chosen hereso that the product of the thickness of each layer and the refractiveindex of the material which composes it is as close as possible to aquarter of the wavelength of the incident signal L1. This feature makesit possible to further increase the efficiency of the mirror 26.However, this value is only indicative, and the result of the productmay be adapted so as to be more or less close to this value according tothe thickness of the first buried insulating layer 21.

Here, the thickness of the portion 220 of the first substrate 22 is thesame as the thickness of the components of the first substrate, notablyof the first waveguide 24 and the photodiode 25. Thus, theimplementation of the Bragg mirror 26 does not require a specific methodstep.

It should be noted that the drawings presented here are simplifiedcross-sectional views. Thus, although the second waveguide 34 and theoptical coupler 36 are represented in the same cross-sectional plane,they may in reality be located in separate planes and/or be oriented indifferent directions.

As illustrated in FIG. 2, the optical (grating) coupler 36 is a singlepolarization coupler, such that a light signal L2 coming from thecoupler and entering the second waveguide 34 is polarized according to asingle polarization state, for example here a transverse electricpolarization, such that a polarization in which the electric fieldcomponent of the light wave is perpendicular to the plane of incidence(also known to the person skilled in the art under the term “Spolarization”).

The optical (grating) coupler 36 is implemented above the Bragg mirror26. Accordingly, a large part of the incident rays passing through thecoupler 36 arrive at the mirror 26 in order to be reflected towards thecoupler 36 and coupled to the waveguide 34. Thus, the signal losses dueto the coupling are reduced. For a wavelength close to one thousandthree hundred and ten (1,310) nanometers, such a mirror exhibits areflectivity of 90%, for an incident wave L1 in transverse electric mode(TE, according to the abbreviation well known to the person skilled inthe art), arriving at an angle θ of 13°.

The second waveguide 34 has a portion implemented above the firstwaveguide 24, and having the same dimensions. Thus, these two parallelportions of the first and second waveguides form an adiabatic couplerfor transferring light from the second waveguide to the first waveguide.

FIG. 3 illustrates an embodiment in which the first portion 220 of thefirst semiconductor film 22 has undergone an additional etching so as tomake it less thick. This thickness may, for example, and advantageously,correspond to the thickness of silicon of some semiconductor portions ofphotonic components etched in the first semiconductor film 22, so thatthe same etching step may be used to form these photonic components andthe Bragg mirror 26. Thus, the Bragg mirror is here formed by the firstportion 220 of the first silicon film 22 having in this example athickness of one hundred and fifty nanometers, by the first portion 230of the layer of dielectric material 23, therefore having a thickness ofthree hundred nanometers, and one portion 310 of the buried insulatinglayer 31 of the second substrate 3, of a thickness of one hundrednanometers.

Thus, the Bragg mirror is optimized for reflecting an incident signal Lhaving here a wavelength close to one thousand five hundred and fifty(1,550) nanometers. For this wavelength, such a mirror exhibits areflectivity of 80%, for an incident wave L1′ in transverse electricmode, arriving at an angle θ of 13°.

FIG. 4 illustrates that the intermediate region INT comprises anadditional silicon layer 27, for example made of polycrystalline siliconor amorphous silicon, of a thickness of one hundred and fiftynanometers, implemented above the first silicon film 22 in order tofurther improve the reflectivity of the Bragg mirror 26.

Here, the portion 220 of the first silicon layer has been etched so asto have a thickness of one hundred and fifty nanometers. It has beencovered with a first portion 231 of the first layer of dielectricmaterial, 23, which has been leveled before the deposition and etchingof the additional silicon layer 27, which has itself been covered with asecond portion 232 of the first layer of dielectric material 23.

Thus, the Bragg mirror in this example comprises four layers:

-   -   the first portion 220 of the first silicon film 22, of a        thickness of one hundred and fifty nanometers,    -   a first portion 231 of the first layer of dielectric material        23, here of a thickness of one hundred and fifty nanometers,    -   the additional silicon layer 27, and    -   the stack of a second portion 232 of the first layer of        dielectric material 23, of a thickness of fifty nanometers and a        portion 310 of the buried insulating layer 31 of the second        substrate 3, of a thickness of one hundred nanometers.

Thus, it is particularly advantageous that the two thicknesses of thepairs of silicon and silicon dioxide layers are identical, whichprovides improved reflectivity. However, it would be conceivable to havea mirror with different thicknesses of layers.

As illustrated in FIG. 5, the optical (grating) coupler is in thisexample a polarization splitting coupler (PSGC, for “PolarizationSplitting Grating Coupler” according to the abbreviation well known tothe person skilled in the art). Nevertheless, this embodiment iscompatible with a single polarization coupler.

Thus, a light signal L1 passing into the coupler 36 will be split intotwo separate polarization subsignals. For example, a first subsignal L3will here be transverse electrically polarized and directed into thesecond waveguide 34, and a second subsignal L4 will be directed into athird waveguide 37 and transverse magnetically polarized (or Ppolarization), meaning a polarization in which the magnetic fieldcomponent of the light wave is perpendicular to the plane of incidence.

It should be noted that the embodiments described here are in no wayrestrictive. Notably, although a Bragg reflector with a thickness offive hundred and fifty nanometers has been described, it is quitepossible to envisage a reflector having a different thickness,preferably but not restrictively with layers whereof the product of thethickness and the refractive index is close to a quarter of thewavelength of the incident signal. The same applies to the number oflayers of the mirror, which may vary with respect to the examplesillustrated in FIGS. 1, 3 and 4.

What is claimed is:
 1. A three-dimensional photonic integratedstructure, including: a support substrate; a first insulating on saidsupport substrate; a first semiconductor film on said first insulatinglayer, wherein the first semiconductor film is patterned to include afirst semiconductor waveguide and a first semiconductor portion; adielectric layer on said first semiconductor film; a second insulatinglayer on said dielectric layer; a second semiconductor film on saidsecond insulating layer, wherein the second semiconductor film ispatterned to include a second semiconductor waveguide and an opticalcoupler which is connected to said second semiconductor waveguide; asemiconductor layer on a first portion of the dielectric layer andcovered by a second portion of the dielectric layer, wherein thesemiconductor layer is insulated from both the first semiconductorportion and the optical coupler, and wherein the semiconductor layer ispositioned between the first semiconductor portion and the opticalcoupler; and a reflective element located below said optical coupler,wherein the reflective element is formed by the first semiconductorportion, the first portion and second portion of the dielectric layer,the semiconductor layer and a portion of the second insulating layer. 2.The structure of claim 1, wherein the first insulating layer and firstsemiconductor film are part of a first silicon on insulator substrate,and wherein the second insulating layer and second semiconductor filmare part of a second silicon on insulator substrate.
 3. The structure ofclaim 1, wherein the first semiconductor portion and the first portionof the dielectric layer form a Bragg mirror.
 4. The structure of claim1, wherein a thickness of the first semiconductor portion and athickness of the first semiconductor waveguide are equal.
 5. Thestructure of claim 1, wherein a thickness of a central portion of thefirst semiconductor portion and a thickness the first semiconductorwaveguide are not equal.
 6. The structure of claim 1, wherein athickness of the optical coupler and a thickness of the secondsemiconductor waveguide are equal.
 7. The structure of claim 1, whereina product of a thickness of the first semiconductor portion and itsrefractive index and a product of a thickness of an overlying portion ofthe dielectric layer and its refractive index are both equal to aquarter of a wavelength of a light signal.
 8. The structure of claim 1,wherein portions of the first semiconductor waveguide and the secondsemiconductor waveguide extend parallel to each other to form anadiabatic coupler for transferring light between the first semiconductorwaveguide and the second semiconductor waveguide.
 9. The structure ofclaim 1, wherein the optical coupler comprises a single polarizationgrating coupler.
 10. The structure of claim 1, wherein the opticalcoupler is a polarization splitting grating coupler.
 11. The structureof claim 1, wherein the reflective element is a Bragg mirror.
 12. Thestructure of claim 1, wherein the first insulating layer and the firstsemiconductor film form a first semiconductor on insulator substrate andwherein the second insulating layer and the second semiconductor filmform a second semiconductor on insulator substrate and wherein the firstsemiconductor on insulator substrate is bonded to the secondsemiconductor on insulator substrate by bonding a top surface of thedielectric layer to a bottom surface of the second insulating layer. 13.The structure of claim 12, wherein the second insulating layer is anoxide layer, and wherein the bonding is a molecular bonding between thedielectric layer and the oxide layer.
 14. A three-dimensional photonicintegrated structure, including: a support substrate; a first insulatinglayer on said support substrate; a first semiconductor film on saidfirst insulating layer, wherein the first semiconductor film ispatterned to include a first semiconductor waveguide and a firstsemiconductor portion; a dielectric layer on said first semiconductorfilm; a second insulating layer on said dielectric layer; a secondsemiconductor film on said second insulating layer, wherein the secondsemiconductor film is patterned to include a second semiconductorwaveguide and an optical coupler which is connected to said secondsemiconductor waveguide; a reflective element located below said opticalcoupler, wherein the reflective element is formed by the firstsemiconductor portion, a portion of the dielectric layer and a portionof the second insulating layer; and wherein the first semiconductorportion, in cross-section, includes an edge portion having a firstthickness and a central portion having a second thickness, wherein thesecond thickness is less than the first thickness.
 15. The structure ofclaim 14, wherein the first insulating layer and first semiconductorfilm are part of a first silicon on insulator substrate, and wherein thesecond insulating layer and second semiconductor film are part of asecond silicon on insulator substrate.
 16. The structure of claim 14,wherein the reflective element is a Bragg mirror.
 17. The structure ofclaim 14, wherein the first thickness of the edge portion and athickness of the first semiconductor waveguide are equal.
 18. Thestructure of claim 14, wherein the second thickness of the centralportion and a thickness the first semiconductor waveguide are not equal.19. The structure of claim 14, wherein a thickness of the opticalcoupler and a thickness of the second semiconductor waveguide are equal.20. The structure of claim 14, wherein a product of a thickness of thefirst semiconductor portion and its refractive index and a product of athickness of an overlying portion of the dielectric layer and itsrefractive index are both equal to a quarter of a wavelength of a lightsignal.
 21. The structure of claim 14, wherein portions of the firstsemiconductor waveguide and the second semiconductor waveguide extendparallel to each other to form an adiabatic coupler for transferringlight between the first semiconductor waveguide and the secondsemiconductor waveguide.
 22. The structure of claim 14, wherein theoptical coupler comprises a single polarization grating coupler.
 23. Thestructure of claim 14, wherein the optical coupler is a polarizationsplitting grating coupler.
 24. The structure of claim 14, wherein thefirst insulating layer and the first semiconductor film form a firstsemiconductor on insulator substrate and wherein the second insulatinglayer and the second semiconductor film form a second semiconductor oninsulator substrate and wherein the first semiconductor on insulatorsubstrate is bonded to the second semiconductor on insulator substrateby bonding a top surface of the dielectric layer to a bottom surface ofthe second insulating layer.
 25. The structure of claim 24, wherein thesecond insulating layer is an oxide layer, and wherein the bonding is amolecular bonding between the dielectric layer and the oxide layer. 26.A three-dimensional photonic integrated structure, including: a supportsubstrate; a first insulating layer on said support substrate; a firstsemiconductor film on said first insulating layer, wherein the firstsemiconductor film is patterned to include a first semiconductorwaveguide and a first semiconductor portion; a dielectric layer on saidfirst semiconductor film; a second insulating layer on said dielectriclayer, wherein the second insulating layer is an oxide layer; and asecond semiconductor film on said second insulating layer, wherein thesecond semiconductor film is patterned to include a second semiconductorwaveguide and an optical coupler which is connected to said secondsemiconductor waveguide; wherein the first insulating layer and thefirst semiconductor film form a first semiconductor on insulatorsubstrate; wherein the second insulating layer and the secondsemiconductor layer form a second semiconductor on insulator substrate;wherein the first semiconductor on insulator substrate is bonded to thesecond semiconductor on insulator substrate by a molecular bonding of atop surface of the dielectric layer to a bottom surface of the oxidelayer of the second insulating layer.
 27. The structure of claim 26,wherein the first semiconductor portion and an overlying portion of thedielectric layer form a Bragg mirror.
 28. The structure of claim 26,wherein a thickness of the first semiconductor portion and a thicknessof the first semiconductor waveguide are equal.
 29. The structure ofclaim 26, wherein a thickness of a central portion of the firstsemiconductor portion and a thickness the first semiconductor waveguideare not equal.
 30. The structure of claim 26, wherein a thickness of theoptical coupler and a thickness of the second semiconductor waveguideare equal.
 31. The structure of claim 26, wherein a product of athickness of the first semiconductor portion and its refractive indexand a product of a thickness of an overlying portion of the dielectriclayer and its refractive index are both equal to a quarter of awavelength of a light signal.
 32. The structure of claim 26, whereinportions of the first semiconductor waveguide and the secondsemiconductor waveguide extend parallel to each other to form anadiabatic coupler for transferring light between the first semiconductorwaveguide and the second semiconductor waveguide.
 33. The structure ofclaim 26, wherein the optical coupler comprises a single polarizationgrating coupler.
 34. The structure of claim 26, wherein the opticalcoupler is a polarization splitting grating coupler.